1. Field of the Invention
The field of Microelectromechanical Systems (MEMS) uses a variety of fabrication technologies, such as surface micromachining developed for the integrated circuit industry, to create highly miniaturized mechanical devices (usually 1 .mu.m to 1 mm in size) on a microelectronic chip. This invention is in the field of microelectromechanical systems (MEMS), and in particular relates to the field of optical MEMS which includes micromirror devices used for a variety of optical applications.
2. Description of the Prior Art
The majority of microelectromechanical systems (EMS) in use today are fabricated in a variety of surface micromachining processes. Surface micromachined devices are formed by the alternate deposition of structural layers to form the device and sacrificial spacer layers to provide mechanical and electrical isolation. Polycrystalline silicon (polysilicon) is the most commonly used structural material and silicon dioxide (oxide) glass is the most commonly used sacrificial material. These layers, formed above a silicon substrate isolated with a layer of silicon nitride, are patterned (using the same advanced photolithography technology employed by the microelectronics industry) to form intricate structures such as motors, gears, mirrors, and various sensors. Cuts made through the oxide layers are used to anchor the upper structural levels to the substrate or to the underlying mechanical structures. At the end of the process, the sacrificial layers are removed using various techniques, such as a hydrofluoric acid release etch, which frees the device to move relative to the substrate. (M. A. Michalicek, J. H. Comtois, and H. K. Schriner, "Design and fabrication of optical MEMS using a four-level, planarized, surface-micromachined polysilicon5 process," Proc. SPIE, Vol. 3276, pp. 48-55, 1998.)
The complexity of the micromachines that can be manufactured in a given process is a function of the number of independent layers of structural material the technology provides. A single independent level of structural material limits designers to simple sensors. Geared mechanisms require two releasable structural layers, a Poly-1 layer to form the gears and a Poly-2 layer to form the locking hub above the ground layer (Poly-0). Motorized geared mechanisms require a minimum of three independent levels. Far more complex mechanisms and systems require even more structural layers.
Surface micromachining fabrication of electronics and MEMS is well developed and widely used both privately and commercially. Countless companies, universities, and government agencies have fabricated micromechanical devices for the last 10 years or more. However, only recently has a fabrication process emerged which offers a unique "pin-joint" feature designed for making the hubs of rotating gear mechanisms. The Sandia Ultra-planar Multi-level MEMS Technology (SUMMiT) process is available through Sandia National Laboratories and offers such a feature for creating locking mechanism that are free to rotate about the joint. This same feature has enabled a novel design of micromirror devices that do not require support flexures (as was the standard) to counteract the force of actuation of the device. As a result, the locking joint supports the mirror surface without a rigid assembly and allows the device to switch between multiple stable positions at high speeds.